All vehicles need some means for allowing the engine to continue turning if the vehicle comes to a stop. Manual transmission vehicles use a clutch, which disconnects the engine from the transmission. Automatic transmission vehicles use a torque converter, a type of fluid coupling that allows the engine to turn somewhat independently of the transmission. Engineers who design automatic transmissions and their components have devised various methods for testing and evaluating them, including torque converters. Evaluating the performance of a torque converter requires that power be supplied to, as well as absorbed from, the torque converter. The use of actual engines as the driving power for such testing is usually considered impractical because of the cost of fuel and maintenance and the problem of controlling emissions. As a result, electric motors are often used as the driving device, but these motors are required to be large and expensive for proper testing and the high accelerations during testing can be damaging to the motor. The absorbing devices are typically dynamometers, which dissipate energy in the form of heat or electricity, and can also be costly.